[PR] avcodec/j2kenc: fix edge case when packet ends on 0xFF (PR #24081)

PR #24081 opened by Niklas Haas (haasn)
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24081
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24081.patch

ISO/IEC 15444-1:2002 (E), Annex B.10.1 "Bit-stuffing routine":

> The last byte in the packet header shall not be an 0xFF value (thus the
> single zero bit stuffed after a byte with 0xFF must be included even if
> the 0xFF would otherwise have been the last byte).

This matches the behavior seen in the jpeg2000 decoder (jpeg2000_flush),
and the behavior of openjpeg.

Sponsored-by: nxtedition AB
Signed-off-by: Niklas Haas <[email protected]>

# Summary of changes

Briefly describe what this PR does and why.

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>From 30bc5a274410ab6ae13806f41f2603eb39f81656 Mon Sep 17 00:00:00 2001
From: Niklas Haas <[email protected]>
Date: Tue, 11 Aug 2026 17:30:21 +0200
Subject: [PATCH] avcodec/j2kenc: fix edge case when packet ends on 0xFF

ISO/IEC 15444-1:2002 (E), Annex B.10.1 "Bit-stuffing routine":

> The last byte in the packet header shall not be an 0xFF value (thus the
> single zero bit stuffed after a byte with 0xFF must be included even if
> the 0xFF would otherwise have been the last byte).

This matches the behavior seen in the jpeg2000 decoder (jpeg2000_flush),
and the behavior of openjpeg.

Sponsored-by: nxtedition AB
Signed-off-by: Niklas Haas <[email protected]>
---
 libavcodec/j2kenc.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/libavcodec/j2kenc.c b/libavcodec/j2kenc.c
index cba4d18adf..11de7524b4 100644
--- a/libavcodec/j2kenc.c
+++ b/libavcodec/j2kenc.c
@@ -175,6 +175,8 @@ static void j2k_flush(Jpeg2000EncoderContext *s)
 {
     if (s->bit_index){
         s->bit_index = 0;
+        if (*s->buf == 0xff)
+            *(++s->buf) = 0;
         s->buf++;
     }
 }
-- 
2.52.0

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